The present invention relates generally to medical devices and more particularly to a delivery system in which the medical device is interposed between a shaft and a sheath.
The use of stents to treat various organs, such as the vascular system, colon, biliary tract, urinary tract, esophagus, trachea and the like, has become common in recent years. Stents are useful in a variety of medical procedures and are often used to treat blockages, occlusions, narrowing ailments and other related problems that restrict flow through a passageway. One common medical procedure in which stents are used involves implanting an endovascular stent into the vascular system. Stents have been shown to be useful in treating numerous vessels throughout the vascular system, including coronary arteries, peripheral arteries (e.g., carotid, brachial, renal, iliac and femoral), and other vessels.
Although stents and other medical devices are used in many different procedures, the use of stents in coronary arteries has drawn particular attention from the medical community because of the growing number of people suffering from heart problems associated with stenosis (i.e., narrowing of the arterial lumen). This has lead to an increased demand for medical procedures to treat such problems. The widespread frequency of heart problems may be due to a number of societal changes, including the tendency of people to exercise less while eating greater quantities of unhealthy foods, in conjunction with the fact that people generally now have longer life spans than previous generations. Stents have become a popular alternative for treating coronary stenosis because stenting procedures are considerably less invasive than other alternatives. Traditionally, stenosis of the coronary arteries has been treated with bypass surgery. In general, bypass surgery involves splitting the chest bone to open the chest cavity and grafting a replacement vessel onto the heart to bypass the blocked, or stenosed, artery. However, coronary bypass surgery is a very invasive procedure that is risky and requires a long recovery time for the patient. To address the growing demand for non-invasive medical procedures for treating coronary arteries and other passageway problems, the medical community has begun to turn away from conventional invasive procedures like bypass surgery and increasingly the treatment of choice now involves various types of stenting procedures.
Many different types of stents and stenting procedures are possible. In general, however, stents are typically designed as tubular support structures that may be inserted percutaneously and transluminally through a body passageway. Traditionally, stents are made from a metal or other synthetic material with a series of radial openings extending through the support structure of the stent to facilitate compression and expansion of the stent. Although stents may be made from many types of materials, including non-metallic materials, common examples of metallic materials that may be used to make stents include stainless steel, nitinol, cobalt-chrome alloys, amorphous metals, tantalum, platinum, gold and titanium. Typically, stents are implanted within an artery or other passageway by positioning the stent within the lumen to be treated and then expanding the stent from a compressed diameter to an expanded diameter. The ability of the stent to expand from a compressed diameter makes it possible to thread the stent through various narrow body passageways to the area to be treated while the stent is in a relatively small, compressed diameter. Once the stent has been positioned and expanded at the area to be treated, the tubular support structure of the stent contacts and radially supports the inner wall of the passageway. As a result, the implanted stent mechanically prevents the passageway from closing and keeps the passageway open to facilitate fluid flow through the passageway.
Particular stent designs and implantation procedures vary widely. For example, stents are often generally characterized as ether balloon-expandable or self-expandable. However, the uses for balloon-expandable and self-expandable stents frequently overlap and procedures related to one type of stent are frequently adapted to other types of stents.
Self-expandable stents are increasingly being used by physicians because of their adaptability to different conditions and procedures. Self-expandable stents are usually made of shape memory materials or materials that act like a spring. Typical metals used in this type of stent include nitinol and 304 stainless steel. A common procedure for implanting a self-expandable stent involves a two-step process. First, the narrowed vessel portion to be treated is dilated with an angioplasty balloon. Second, the stent is implanted into the dilated vessel portion. However, other procedures are also possible, such as adding an additional dilation step after stent implantation or implanting a stent without dilation. To facilitate stent implantation, the stent is installed on the end of a catheter in a small, compressed state. The stent is usually retained in the compressed state by inserting the stent into a sheath at the end of the catheter. The stent is then guided to the portion of the vessel to be treated as described above. Once the catheter and stent are positioned adjacent the portion to be treated, the stent is released by pulling, or withdrawing, the sheath rearward. Normally, a step or other feature is provided on the catheter to prevent the stent from moving rearward with the sheath. After the stent is released from the retaining sheath, the stent radially springs outward to an expanded diameter until the stent contacts and presses against the vessel wall. Traditionally, self-expandable stents have been used in a number of peripheral arteries in the vascular system due to the shape memory characteristic of these stents. One advantage of self-expandable stents for peripheral arteries is that traumas from external sources do not permanently deform the stent. Instead, the stent may temporarily deform during an unusually harsh trauma but will spring back to its expanded state once the trauma is relieved. However, self-expandable stents may be used in many other applications as well.
The above-described examples are only some of the applications in which intravascular devices are used by physicians. Many other applications for intravascular devices are known and/or will be developed in the future. For example, similar intravascular procedures are also used to implant stent-grafts to treat aneurysms, to deploy vascular filters, and to implant artificial valves in veins.
Typically, intravascular devices are provided with markers and/or pushing members that are attached or formed along the proximal and/or distal ends of the device structure. These features may be used for a number of purposes and often serve more than one function. For example, markers are usually provided at both the proximal and distal ends of the intravascular device to assist the physician in positioning the device during intravascular procedures. Generally, separate markers are needed on most intravascular devices since the structure of the device itself usually cannot be seen easily on x-ray or other visualization equipment. This is due in part to the types of material that are usually used in intravascular devices and the slenderness of the structural members often used in the structure of the device. Markers address this visualization problem by providing features with increased radiopacity along the proximal and distal ends of the device. Typically, the markers are usually filled with a radiopaque material like gold or platinum. As a result, the radiopaque material in the markers can be seen more easily on the physician's visualization equipment than the structure of the device itself.
Pushing members are also used at the proximal and/or distal ends of many intravascular devices. In general, the pushing members provide a contact surface that allows the device to be moved by pushing against the pushing members. Pushing members may be used at several different stages. For example, during the manufacture of expandable devices and the corresponding delivery systems, the intravascular device is usually loaded into the delivery system in a compressed state. Many different types of delivery systems for intravascular devices are known to those in the art, and therefore a detailed description is unnecessary. However, as described above, delivery systems often include a retaining sheath at the end of a catheter that restrains the outer surface of the device and keeps the device compressed until the intravascular device is released within the patient's body. A common manufacturing method for loading intravascular devices into the retaining sheath involves compressing the device while at the same time pushing on one end of the device in order to slide the device into the sheath. Alternatively, the device may be compressed and pushed into a transfer tube first and then pushed again through the transfer tube into the delivery system.
Pushing members are also used on the proximal end of intravascular devices in order to release the device from the delivery system. As previously described, intravascular devices may be released by pulling the retaining sheath off the device. Typically, the delivery system also includes a step or other structure within the retaining sheath which contacts the proximal end of the device. Generally, the step and the sheath are designed to move relative to each other so that as the sheath is pulled back, the step is maintained in place. As a result, the step prevents the device from moving rearward with the retaining sheath as the sheath is pulled back. In effect, the device is pushed forward relative to the sheath which is pulled rearward.
Typically, the markers on an intravascular device are also used as pushing members and vice versa. Normally, the markers are used as pushing members by pushing on the end surface of the markers. However, at least in the case of stents, current markers may be inadequate when used as pushing members. This can be a particular problem on longer length stents. Generally, most stents that are currently used for medical treatments are 8 cm or less in length. However, stents that are longer than 8 cm are becoming more desirable to treat various peripheral arteries, such as the superficial femoral artery. When longer stents like these are pushed, either during loading into the delivery system or during release, higher frictional forces must be overcome in order to move the stent. The longer length of some of these stents also makes the stent generally less stable than shorter stents. As a result, buckling and deformation problems that may occur when pushing on a stent tend to be more pronounced and damaging on longer stents. However, these types of problems may exist with any stent.
One problem with current delivery systems for intravascular devices is that it can be difficult to precisely release the device within a body cavity. For example, in the case of a body lumen, such as an artery, it is often important to precisely position the intravascular device longitudinally within the artery. One particular example where physicians may be concerned about this issue is the placement of a stent within an artery to treat a stenosed area. Typically, the length and diameter of the stent is chosen by the physician based on the particular artery being treated and the area of stenosis. However, on occasion the physician may encounter difficulties during the intravascular procedure which results in the stent being implanted at a less than optimal location. For example, the stent may be released either longitudinally distal or proximal to the desired placement. As a result, the stenosed area may not be completely covered by the stent. If this occurs, the physician may need to perform a second procedure to implant a second stent to cover the remaining untreated stenosed area. This results in greater costs and increased risks due to the extra procedures involved. In addition, the desirable restenosis of the artery wall that usually encapsulates the stent after implantation may not fully encapsulate the overlapped portion of the stents. Moreover, when metal stents are used, the overlapping contact between the two stents may corrode and cause an allergic reaction in the patient.
Precise positioning of intravascular devices can be important in a variety of other procedures and for other reasons as well. For example, it can also be important to precisely position stents or other intravascular devices when the device is being released near an artery bifurcation or near another organ or structure. Procedures related to treating aneurysms also may require precise positioning of a device. Precise positioning of an intravascular device may be made more difficult for a number of reasons. One problem that physicians have encountered is a tendency of self-expandable stents and similar devices to “jump” out of the delivery device during release. This may occur when the distal end of the stent is initially released and the proximal end of the stent is still within the delivery system. Since self-expandable stents are elastic and seek an expanded diameter, the distal end of the stent may rapidly expand in this situation and unexpectedly pull the proximal end of the stent out of the delivery system. This can make positioning of the stent difficult to control. The control mechanism may also make precise positioning difficult. Normally, in the case of self-expandable stents, the control mechanism includes a first handle attached to the sheath and a second handle attached to a core and/or step. Typically, the physician releases the stent by holding the second handle in a fixed position while pulling the first handle rearward. This causes the sheath to move rearward relative to the core and/or step. The step prevents the stent from moving rearward with the sheath, and as a result, the stent is released by the withdrawal of the sheath. However, the physician may find it difficult to precisely control the movement between the two handles, which may cause the stent or other device to be released in a less than optimal position.
Another problem with conventional delivery systems is that they are usually not adapted to implant more than one intravascular device at a time. However, in many procedures multiple devices may need to be released within the body cavity being treated. As a result, the physician may need to introduce and withdraw several different delivery systems through the patient's passageways to complete the procedure. The disadvantage of this is that each time a delivery system must be introduced and withdrawn damage may occur to the passageways. The overall length of the procedure and the cost of the procedure is also increased.
Accordingly, it is apparent to the inventor that an improved delivery system for medical devices is needed. A solution to these and other problems is described more fully below.